Hard disk drives incorporate magnetic storage disks and read/write heads which are capable of reading data from and writing data onto the rotating storage disks. Data is typically stored on each magnetic storage disk in a number of concentric tracks on the disk. The read/write heads, also referred to as read/write transducers or read/write elements, are integrated within a slider. The slider, in turn, is part of an actuator assembly which positions the heads relative to the surface of the storage disks. This may be at a predetermined height above the corresponding storage disk. The actuator assembly is typically controlled by a voice coil motor which acts to position the slider over the desired track. One or more read/write heads may be integrated within a single slider. In the case of non-contact sliders, a cushion of air is generated between the slider and the rotating disk. The cushion is often referred to as an air bearing.
Hard disk drives are an efficient and cost effective solution for data storage. Depending upon the requirements of the particular application, a disk drive may include anywhere from one to eight or more hard disks and data may be stored on one or both surfaces of each disk. While hard disk drives are traditionally thought of as a component of a personal computer or as a network server, usage has expanded to include other storage applications such as set top boxes for recording and time shifting of television programs, personal digital assistants, cameras, music players and other consumer electronic devices, each having differing information storage capacity requirements.
A primary goal of disk drive assemblies is to provide maximum recording density on the storage disk. In order to provide greater storage capacity on a storage disk, track widths have become increasingly narrow. However, decreasing the width of tracks makes it more difficult for the read/write heads to accurately read and write information to and from the tracks. Not only is it difficult to physically position the read/write element over a narrow width track, but it is increasingly difficult to maintain the read/write element over the track at an optimal position for accurate data transfer. Air turbulence created by the spinning disks, disk flutter and spindle vibrations, temperature and altitude can all adversely affect registration of the read/write element relative to the tracks. Moreover, increasing the speed of the rotating disks to achieve increased data access times increases air turbulence, which increases misregistration between the read/write element and the tracks on the storage disks (track misregistration or TMR). Higher rotational speeds can also increase disk flutter and spindle vibrations further increasing TMR.
Accuracy can be further adversely affected if the read/write heads are not maintained within an optimum height range above the surface of the storage disk. Thus, a related goal is to increase reading efficiency or to reduce reading errors, while increasing recording density. Reducing the distance between the magnetic transducer and the recording medium of the disk generally advances both of those goals. Indeed, from a recording standpoint, the slider is ideally maintained in direct contact with the recording medium (the disk) to position the magnetic transducer as close to the magnetized portion of the disk as possible. Contact positioning of the slider permits tracks to be written more narrowly and reduces errors when writing data to the tracks. However, since the disk rotates many thousands of revolutions per minute or more, continuous direct contact between the slider and the recording medium can cause unacceptable wear on these components. Excessive wear on the recording medium can result in the loss of data, among other things. Excessive wear on the slider can result in contact between the read/write transducer and the disk surface resulting, in turn, in failure of the transducer, which can cause catastrophic failure.
Similarly, the efficiency of reading data from a disk increases as the read element is moved closer to the disk. In particular, because the signal to noise ratio increases with decreasing distance between the magnetic transducer and the disk, moving the read/write element closer to the disk increases reading efficiency. As previously mentioned, the ideal solution would be to place the slider in contact with the disk surface, but there are attendant disadvantages. In non-contact disk drives there are also limitations on how close a read/write element may be to the surface of a disk. A range of spacing is required for several reasons, including the manufacturing tolerances of the components, texturing of the disk surface and environmental conditions, such as altitude and temperature. These factors can cause the read/write element flying height to vary or even cause the read/write element to contact the spinning disk.
Disk drives are assembled in a clean room to reduce contamination from entering the drive prior to final assembly. Thus, the air that is trapped within the drive once it is finally sealed is filtered room air. Accordingly, seals used in disk drives between the base plate and cover are designed to prevent contaminants from entering the drive. Such seals are not designed to prevent internal air and other gases from exiting through the seal and out of the drive. Loss of gas through the seals is anticipated and accommodated by use of a filtered port to maintain air pressure within the drive at the pressure of the air outside of the drive.
As an alternative to air-filled drives, advantages may be achieved by filling disk drives with gases having a lower density than air. For example, Helium has a lower density than air at similar pressures and temperatures and can enhance drive performance. When compared with air, lower density gases can reduce aerodynamic drag experienced by spinning disks within the drive, thereby reducing power requirements for the spindle motor. A Helium-filled drive thus uses substantially less power than a comparable disk drive that operates in an air environment. Relatedly, the reduction in drag forces within the Helium filled drive reduces the amount of aerodynamic turbulence that is experienced by the drive components such as the actuator arms, the suspensions and the heads. Reduction in turbulence allows drives filled with low density gas to operate at higher speeds compared with air-filled drives, while maintaining the same flying height and thereby maintaining the same range of read/write errors. Low density Helium drives also allow for higher storage capacities through higher recording densities due to the fact that there is less turbulence within the drive, which allows the tracks to be spaced more closely together.
In order to maintain the gas in the drive, low density Helium drives must be permanently sealed. Accordingly, there is no filtered port to equalize pressure within the drive as with air-filled drives. As a result, the seal between the cover and base plate must prevent leakage and maintain a threshold level of low density gas within the sealed environment over the expected lifetime of the drive. However, light or low density gases are difficult to contain within a disk drive enclosure due to diffusion of the gas and problems with sealing the drive. Therefore, it is difficult to prevent the low density gas from escaping from the sealed drive environment. Gas that is lost may or may not be replaced with ambient air. In either case, the performance of the drive will change from the design specifications, namely, a low density Helium sealed environment. For example, as a result of low density gas leaks from the drive, the flying height of the heads is altered, increasing the likelihood of data loss.
If the low density gas leaks out of a drive and is replaced by air, the increased concentration of air may increase the turbulent forces on the heads due to the increased drag forces within the drive and may cause the heads to fly at too great a distance above the disks, thereby increasing instances of read/write errors. The replacement of a light gas with air also increases the amount of power required by the spindle motor to rotate the disks, because of the resulting increase in aerodynamic drag. If the light gas leaks from the drive and is not replaced by air, the heads may fly at a distance too close or in contact with the disks, thereby increasing instances of read/write errors as well as damage to the disk surface and head and data loss due to contact between the disk and head (i.e., head crashes). The risk of unanticipated failure due to inadequate amounts of low density gas within the drive is a drawback to low density Helium drives. Indeed, data stored within the drive may be irretrievably lost if the drive fails due to the loss of the low density gas environment.